No safe dose

The recent zeal among conservative politicians for expanding Australia’s nuclear industry should raise questions about its potential impact on the health of humans and their habitat. Unfortunately, the recently released Switkowski Report (pdf 3.63MB) on Uranium Mining Processing and Nuclear Energy brings little serious critical analysis to bear on the subject.

We exist in a naturally radioactive environment: the rocks and mountains, the sun in particular, produce a “background” level. Average exposure to “background” ionizing radiation worldwide is measured at 2.4 millisievert (mSv) a year. About half of this is from radon gas and its decay products.

However, human activities in the past century have greatly increased our exposure to ionizing radiation, through atomic weapons development, testing and use, as well as uranium-mining and nuclear electricity generation. The ongoing atmospheric fallout from the nuclear weapons testing in the 50s and 60s adds an average extra dose to us all of 0.02mSv per year.

These doses are estimated to have already resulted in 430,000 additional fatal cancers worldwide by the year 2000, and a total of 2.4 million extra cancer deaths long-term.

Unfortunately there is no level of radiation exposure below which we are at zero risk: even low-level medical exposures such as chest X-rays (0.04mSv per test) carry a quantifiable risk of harm. While high doses of ionizing radiation will cause greater health damage, even low doses are associated with adverse environmental and human consequences.

Using the “linear no-threshold” risk model, the 2005 US National Academy of Sciences Committee on the Biological Effects of Ionizing Radiation (BEIR VII) estimated:

• over a lifetime, a dose of 1mSv creates an excess risk of cancer of approximately 1 in 10,000. Higher doses are associated with proportionately higher risk, for example a dose of 100mSv would cause 1 in 100 people to develop cancer;
• approximately 1 individual in 100 persons would be expected to develop cancer from a lifetime (70 years) exposure just to background x and gamma rays (excluding radon and other high LET radiations)

It should be noted that while these are average risks, the risks in vulnerable groups of the population may be considerably higher. BEIR VII assessed women as having about twice the radiation risk for solid cancer incidence as men, and 38 per cent higher cancer mortality risk than men.

Children are at even greater risk - radiation during infancy for boys results in three to four times the cancer risk as between 20 to 50 years of age, and female infants have double the risk of boys.

Ionizing radiation causes damage to the DNA in living cells. Atoms and molecules become ionized or excited, which can produce free radicals, break chemical bonds, produce new chemical bonds and cross-linkage between macromolecules and damage molecules that regulate vital cell processes such as DNA, RNA and proteins.

In recent years biologists have identified specific radiation-induced damage at the molecular level to nucleotide sequences on chromosomal DNA, including double-strand breaks, large deletions and sister chromatid exchange.

The cell can repair certain levels of damage in its chromosomal DNA: at low doses cellular damage is usually repaired. However, faulty repairs may lead to cell death or to proliferation of abnormal cells which form a cancer.